1. Field of the Invention
The invention pertains to a bridging clutch for a hydrodynamic clutch device enclosed by a housing having a friction surface, a piston which can move axially with respect to the housing, the piston having a friction surface which faces the friction surface of the housing, and a pretensioning element supported axially between the piston and the housing for actuating the piston in a predetermined axial direction.
2. Description of the Related Art
A bridging clutch for a hydrodynamic clutch device provided with a plurality of disks to form friction surfaces between a cover of the housing of the hydrodynamic clutch device and a piston is known from U.S. Pat. No. 4,177,885. The piston is mounted nonrotatably but with freedom of axial movement by way of its base on the hub of a turbine wheel and is subjected to pretension in the direction toward the disks by a pretensioning element in the form of an axial spring, which is supported against the turbine hub and thus against a component of the hydrodynamic clutch device.
When the bridging clutch is subjected to positive pressure in the hydrodynamic circuit and thus in the area between the turbine wheel and the piston, the piston makes contact with the disks and thus activates the bridging clutch. During this process, which is referred to as “engagement” in the art, the pretensioning element assists the movement of the piston, as a result of which the engaging operation can be accelerated and can also be accomplished even if the difference between the pressure in the hydrodynamic circuit and that in the pressure space located axially between the piston and the cover of the housing is very small. If the bridging clutch is to be disengaged, however, which is accomplished by means of a positive pressure in the pressure space with respect to the hydrodynamic circuit, the piston must be moved against the force of the pretensioning element toward the turbine wheel. Although this movement of the piston, referred to as “disengagement” in the art, must be accomplished against the force of the pretensioning element, the increased difficulty of performing the disengaging operation thus caused is tolerated in view of the advantages associated with the engaging operation.
The arrangement of the pretensioning element on the turbine hub and thus at a location far inside the disks in the radial direction is likely to be problem in the bridging clutch according to U.S. Pat. No. 4,177,885, in the sense that the pretensioning element must act on the disks via the piston, which thus serves as a speed reducer. If the piston has elasticity in the axial direction, furthermore, the axial force exerted by the pretensioning element is transmitted to the disks in such a way that the disks are subjected to uneven load. It is therefore impossible to avoid the at least partial premature wear of the disks.
Another disadvantage of this bridging clutch is to be found in its axial connection to the turbine wheel by way of the pretensioning element. Axial displacements of the turbine wheel, such as those which occur during the transition between push and pull phases, for example, are accordingly transmitted via the pretensioning element to the piston and thus influence the function of the bridging clutch.
U.S. Pat. No. 5,826,690 discloses a bridging clutch having a plurality of tangential leaf springs, which are attached at one end to the cover of the housing of a hydrodynamic clutch device and at the other end to the piston, where the latter attachment is achieved by means of lock-ring or collar bolts, which offer the advantage that the tangential leaf springs can be attached to the piston from a certain side during the assembly procedure. The tangential leaf springs, however, serve only to connect the piston nonrotatably to the cover and thus to the housing of the hydrodynamic clutch device; they do not exert any pretension in a predetermined direction.
As a result of the absence of an axially operative pretensioning element, there is the problem that bridging clutches of this type take a comparatively long time to react during the clutch-engaging operation, this reaction time being easily on the order of 300–600 ms. It is often desirable, however, to have a much shorter engagement time on the order of, for example, approximately 150 ms. Another disadvantage is to be found in the loss of ability to control the piston, because, when the piston is disengaged, its position can be unstable if there is only a small pressure difference between the pressure space and the hydrodynamic circuit. A considerable amount of effort is required to compensate for this situation, to the extent that it can be compensated at all. An instability of this type can have the effect that the piston is shifted abruptly and undesirably in the engaging direction, which can be perceived negatively in the vehicle as a jerk.